It has been established that several intrinsic and extrinsic stimuli, including growth factors and mechanical load, interact with the genetic program f the heart to regulate normal and pathological cardiac differentiation and growth. Mechanical loading of cardiac myocytes may have an intrinsic origin as a result of the force of myocyte contraction and may also result from the extrinsic forces associated with the hemodynamic effects of blood flow. Both contraction of cardiac myocytes and blood flow begin at an early stage in development. Mechanical load is known to increase the paracrine release of growth factors which also increases the rate and force of myocyte contraction. Mechanical loading and some growth factors further influence cardiac myocyte and interstitial tissue growth and differentiation which are primary factors in determining heart development and function. This grant proposes to utilize mice in culture systems which will examine the relationships between myocyte differentiation, mechanical loading and two growth factors, angiotensin II (Ang II) and platelet derived growth factor (PDGF). The primary goals of this proposal are to experimentally manipulate early embryonic mice by the addition of Ang II and PDGF to cultures of normal and transgenic animals and cells to determine the role of these growth factors in early cardiac development. The specific aims are to: 1) establish the mouse whole embryo culture system in our laboratory and to determine the temporal and spatial effects of Ang II and PDGF on normal and transgenic mouse cardiac myocyte and heart differentiation; 2) use the aligned myocyte culture system to apply mechanical tension to cultured normal and transgenic mouse myocytes to determine the interaction between Ang II, PDGF and mechanical load; and 3) develop an objective morphometric assay to assess myocyte shape and myofibrillar structure and organization.